Methods and apparatus for cutting, grinding and polishing single-crystal workpieces

ABSTRACT

Mechanical vibrations generated during the cutting, grinding and polishing of single-crystal material, such as diamonds, are converted into electrical signals by a transducer; amplified; and used to control the rotation of the workpiece holder. The control is such that the single-crystal material is oriented along its &#39;&#39;&#39;&#39;easy&#39;&#39;&#39;&#39; axis, for ease of cutting, etc.

United States Patent Weinz [451 Mar. 21, 1972 METHODS AND APPARATUS FOR CUTTING, GRINDING AND POLISHING SINGLE-CRYSTAL WORKPIECES Inventor: Ernst August Weinz, ldar-Oberstein, Germany Assignee: Ernst Fr. Weinz, ldar-Oberstein, Germany Filed: Apr. 10, 1970 Appl. No.: 27,229

Foreign Application Priority Data Apr. 11, 1969 Germany ..P l9 18 347.2

US. Cl. ..5l/l65 R, 51/283 Int. Cl ..B24b 49/00 Field oiSearch ..51/165 R, 125, 229, 283;

[56] References Cited UNITED STATES PATENTS 1,160,843 1/1916 Loesser ..5l/125 2,023,494 12/1935 Strieby.... ...51/16S R 2,340,843 2/1944 Bailey ...5l/283 X 3,520,088 7/ 1970 Leibowitz ..5 H229 X Primary Examiner-Lester M. Swingle Attorney-Kurt Kelman [57] ABSTRACT Mechanical vibrations generated during the cutting, grinding and polishing of single-crystal material, such as diamonds, are converted into electrical signals by a transducer; amplified; and used to control the rotation of the workpiece holder. The control is such that the single-crystal material is oriented along its easy axis, for ease of cutting, etc.

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SHEET 4 OF 4 INVENTOR. ERNST A. WE/NZ BACKGROUND OF THE INVENTION This invention relates to the cutting, grinding and polishing of single-crystal materials, such as diamonds, sapphire and the like. More specifically, in a preferred embodiment, the invention relates to methods and apparatus for automatically rotating the single-crystal material with respect to the cutting or grinding tool, so that the single-crystal material attains a preferred orientation. 1

As is well known, a single-crystal material is a material in which all the constituent parts of the material exhibit the same crystallographic orientation. Such single-crystal materials may occur naturally or they may be produced artificially. Diamond is an example of a naturally occurring, single-crystal material, and the synthetic saphire which is used to manufacture phonograph stylus is an example of an artificially-grown single crystal material.

Because the crystallographic axes of the constituent parts of a single-crystal material are all uniformly oriented, such materials are inherently anisotropic. That is to say, their electrical, optical and mechanical properties vary according to the orientation of the material.

Accordingly, in the cutting, grinding or polishing of singlecrystal material, it is advantageous to orient the workpiece so that the cutting or grinding takes place in the direction of least mechanical resistance, i.e., along the so-called easy axis of the material. In the case of a diamond, this easy" axis is parallel to the cubic edge, i.e., parallel to the main crystallographic axis of the material. If this preferred orientation is not attained and the cutting or grinding is performed along the socalled "hard axis, the diamond will have sharp, jagged edges and it will be impossible to obtain a smooth, polished facet. Unfortunately, it is not an easy matter to determine which is the easy axis of a single-crystal material and skilled and highly paid labor must be employed to make this determinatron.

To eliminate this requirement, one prior art device continuously alters the direction of grinding so that it coincides, at least part of the time, with the easy axis of the material being ground. Such devices have not proved successful in practice, however, because, for a cubic face, the angle of aperture is only :4, for example, and thus the optimal orientation of the single-crystal material only coincides with the direction of grinding for a small percentage of the total operating time.

In another prior art machine, a skilled craftsman makes an initial alignment of the workpiece, but the actual grinding operation is performed by semi-skilled labor. Such machines have also not proved to be too successful in practice, as the initial alignment is time-consuming and there is no guarantee that the alignment will not shift during the grinding operation.

The problem then is to find methods and apparatus for automatically orienting the workpiecein the preferred direction, during the grinding process, without the necessity of employing, skilled and highly paid machine operators.

This problem has been solved, in the instant invention, by a method of adjusting the orientation of a single-crystal workpiece, with respect to a cutting tool, so that said workpiece attains a preferred orientation, comprising the steps of generating an electrical signal representative of the difference between the present orientation of the workpiece and said preferred orientation and rotating said workpiece, with respect to said tool, under control of said electrical signal, until the workpiece attains said preferred orientation.

One illustrative apparatus for practicing the above method comprises means for supporting said workpiece proximate said cutting tool; a vibration-sensitive pickup device mounted to said supporting means for converting mechanical vibrations established therein by the interaction of said tool and said workpiece into an electrical signal, the magnitude of said electrical signal being proportional to the difference between the present orientation of the workpiece and the preferred orientation; and means, responsive to the magnitude of said electrical signal, for rotating said workpiece towards said preferred orientation.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a side view of an illustrative apparatus which may be used, according to this invention, to automatically rotate a single-crystal workpiece with respect to a grinding tool, so that the workpiece attains a preferred orientation;

FIG. 2 is a front view of the apparatus shown in FIG. 1; FIG. 3 is a plan view of the apparatus shown in FIGS. I and FIG. 4 is a schematic diagram of an illustrative amplifier circuit which may advantageously be used, in accordance with this invention, to amplify feedback signals from the apparatus shown in FIGS. 1-3;

FIG. 5 is a schematic diagram of an illustrative control cir cuit which may advantageously be used with the amplifier shown in FIG. 4 to control the apparatus shown in FIGS. 1-3; and

FIG. 6 is a schematic diagram of a modified control circuit.

DETAILED DESCRIPTION OF THE INVENTION This invention is based upon the discovery that sonic vibrations are created whenever a cutting or grinding tool is operative on a single-crystal workpiece, such as diamond or saphire, and on the further discovery that these vibrations vary in magnitude according to the relative orientation of the easy" and hard axes of the workpiece, with respect to the cutting or grinding tool.

Thus, in one embodiment of the invention, a pickup device is mounted upon the cutting machine and the amplified output of the pickup is used to control the rotation of the workpiece holder, in an offsetting manner, so as to minimize the output of the pickup, which condition coincides with the desired orientation of the easy" axis of the workpiece material.

Referring now to FIGS. 1, 2 and 3, one illustrative embodiment of the invention comprises a base member 9 to which a hollow cylindrical member 3 is secured.

The workpiece 2 to be ground, or cut, is fastened to the lower end of a shaft (not shown) which is rotatably mounted within cylindrical member 3. The upper end of the rotatable shaft is connected to an elastic coupling 4 which, in turn, is connected to the shaft of a DC motor 5.

Motor 5 is fastened to a clamping member 6 which is also connected to base member 9. A transducer or pickup device 7 is fastened to clamping member 6 so that the pressure sensitive end thereof is proximate the upper surface of base member 9.

Base member 9 rests upon a support 12 in such a manner that the tip of workpiece 2 contacts the surface of a grinding disk 1, which typically may comprise a sintered mass of cobalt and diamond or a disk which has been impregnated with diamond dust. A spirit level 8 is affixed to the upper surface of base member 9 and adjustment of the axis of rotation of the workpiece, with respect to the plane of rotation of grinding disk 1, may be accomplished in both the horizontal and vertical planes, by adjustment of a pair of leveling screws 11-11 and lock nuts 10-10, while observing spirit level 8.

Referring now to FIG. 4, the output of pickup device 7 is connected to the input of a low-frequency amplifier comprising a pair of substantially identical amplifying stages; the first including transistors T and T the second including transistors T and T More specifically, the signal from pickup device 7 is applied through a capacitor C to the base of transistor T which is operating in the common-emitter mode. The amplified output of transistor T is developed across a collector resistor R5 and fed to the base of transistor T which is also operating in the common-emitter mode. The amplified output of transistor T is developed across collector resistors R and R and fed, through capacitor C, to the base of transistor T at the input of the second amplifying stage.

In an analogous manner, the alternating signal from pickup 7 is amplified in the second stage and is developed across the collector of transistor T,. This amplified signal is then applied to the base of a transistor T through a capacitor C Transistor T is so biased that, in addition to amplifying the signal from transistors T,-T.,, the signal from pickup device 7 is rectified into a DC signal which DC signal is passed to the input of a control circuit, shown in FIG. 5.

Referring to FIG. 5, the control circuit comprises a Schmitt- Trigger, including transistors T, and T an amplifying stage including transistor T and a relay D,. As is well known, a Schmitt Trigger is a circuit which produces an output voltage of constant peak value (a flat-topped pulse) whenever the input waveform exceeds a specific voltage. Thus, whenever the DC voltage supplied from the output of the amplifier-detector, shown in FIG. 4, exceeds a predetermined value, an output voltage is developed across resistor R25 in the collector circuit of transistor T,.

This voltage, in turn, is fed, via a resistor R to the base of an amplifying transistor T The armature winding of a relay D, is connected in the collector circuit of transistor T,,. Thus, whenever transistor T, is biased on by the output of the Schmitt Trigger (T and T relay D, operates to apply potential to the armature winding ofa motor M1 (5 in FIGS. 1-3).

As previously discussed, pickup device 7 generates a lowfrequency AC signal when energized by mechanical vibrations generated by the system including the tool, the workpiece and the machine, during the grinding of a single-crystal material. The amplitude of the mechanical vibrations generated in the single crystal material depends upon its direction of growth. Taking the diamond, as an example, if the direction of growth is correct, i.e., if the easy orientation is as closely parallel as possible to the main crystallographic axis of the diamond, the amplitude of the mechanical vibrations generated is lower than would be the case for a less favorable direction of growth. As previously mentioned, such unfavorable directions of growth are called the hard orientation and can only be machined with difficulty. More specifically, unfavorable directions of growth are those which are spatially oriented close to the diagonal between the main axes of the crystal.

In operation, the single-crystal material to be cut ground, or polished, is placed within the holder and the grinding or cutting wheel rotated. The sonic vibrations which are generated in the workpiece, tool and machine are transmitted to pickup 7. These mechanical vibrations are transformed into an alternating potential by the pickup and fed to the input of transistor T, in the amplifier circuit (FIG. 4).

Assume that the single-crystal material is oriented improperly; that is, along its hard axis. The resulting mechanical vibrations which are transmitted to the pickup will have a relatively large amplitude. The AC output from the pickup will also be relatively large and when amplified by transistors T,-T, and rectified by transistor T, will generate an input to the Schmitt Trigger (T and T FIG. 5), which will exceed the threshold level of the Schmitt Trigger.

As a result, transistor T, will be turned on and relay D, energized. This, in turn, will apply an energizing potential to motor M, (5; FIG. 1).

When so energized, the motor will slowly rotate the workpiece about its axis, altering the orientation of the easy" axis with respect to the plane of the grinding wheel. As the motor continues to rotate the workpiece, the orientation of the easy" axis becomes closer and closer to being parallel to the plane of the cutting or grinding wheel. Thus, the magnitude of the mechanical vibrations generated in the system, and hence the input of the AC signal applied to amplifying transistors llj radually falls lower and lower.

Eventually a point is reached where the amplitude of the signals applied to the input of the Schmitt Trigger circuit no longer exceeds the threshold level thereof. When this occurs transistor T, is turned off" and relay D, is released. This, in turn, disconnects the energizing potential to motor M, and thereby terminates further rotation of the workpiece. If the system is properly adjusted, this will also coincide with the condition where the easy axis of the workpiece is exactly parallel to the plane of the cutting or grinding tool.

Variable capacitor C, and variable resistor R,, which are connected in the input circuit of transistor T,, may be adjusted to control the overall sensitivity of the Schmitt Trigger. Thus, the point, at which transistor T, is turned off and relay D, released may be made to coincide with the point of time when the easy axis of the workpiece is exactly parallel to the axis of the grinding wheel. These adjustable elements may also be used to compensate for variations in the magnitude of the input signals to the amplifier caused by different speeds of rotation for the grinding disc, different workpiece material, etc, etc.

The methods andapparatus of this invention may also be used to grind spherical or conical single-crystal workpieces to extremely fine tolerances as the automatic positioning mechanism will always act to position the workpiece for grinding in the most favorable orientation.

One skilled in the art will appreciate that the invention disclosed herein may easily be adapted to automatically orient the single-crystal workpiece in the hard direction rather than the easy direction, if this is desired. This would be the preferred direction, for example, if the single-crystal material were to be the tool and the grinding wheel, or some substitute therefor, were to be the workpiece. Clearly, in that case, orienting the single-crystal material in the "hard direction would extend the life of the tool and ensure smooth, clean cuts in what is now the workpiece.

Fortunately, this modification may easily be accomplished. One example, as shown in FIG. 6, would be to simply connect motor M, to a pair of nonnally made contacts on relay D,, rather than to the normally open contacts shown in FIG. 5. If this were done, motor M, would be deenergized only when relay D, was released, which would occur only when the magnitude of the amplified signal from pickup device 7 exceeded the threshold of the Schmitt-Trigger. This, of course, would occur when the single-crystal material produced its maximum vibrational output i.e., when it was oriented in the hard direction.

One skilled in the art may make various changes and modifications to the methods and apparatus disclosed herein without departing from the spirit and scope of the invention.

What is claimed is:

1. A method of adjusting the orientation of a single-crystal workpiece, with respect to a cutting tool, so that said workpiece attains a preferred orientation, comprising the steps of:

generating an electrical signal representative of the difference between the present orientation of the workpiece and said preferred orientation; and

rotating said workpiece, with respect to said tool, under control of said electrical signal, until the workpiece attains said preferred orientation.

2. The method according to claim I, wherein the magnitude of said electrical signal decreases as the orientation of said workpiece approaches the preferred orientation; the method comprising the further steps of:

amplifying said electrical signal in a control circuit including a bistable device having a predetermined threshold; and

terminating the rotation of said workpiece, under control of said bistable device, whenever the magnitude of said amplified electrical signal falls below said predetennined threshold.

3. The method according to claim 2, comprising the further step of:

adjusting the amplifying sensitivity of said control circuit to compensate for variations in the magnitude of said electrical signal caused by variations in the relative movement of said workpiece and said tool; and by variations in the mechanical properties of said workpiece and said tool. 4. The method according to claim 1, wherein the magnitude of said electrical signal increases as the orientation of said workpiece approaches the preferred orientation; the method comprising the further steps of:

amplifying said electrical signal in a control circuit including a bistable device having a predetermined threshold; and

terminating the rotation of said workpiece, under control of said bistable device, whenever the magnitude of said amplified electrical signal exceeds said predetermined threshold,

5. The method according to claim 4, comprising the further step of:

adjusting the amplifying sensitivity of said control circuit to compensate for variations in the magnitude of said electrical signal caused by variations in the relative movement of said workpiece and said tool; and by variations in the mechanical properties of said workpiece and said tool.

6. Apparatus for automatically rotating a single crystal workpiece, with respect to a cutting tool, so that said workpiece attains a preferred orientation, which comprises:

means for supporting said workpiece proximate said cutting tool;

a vibration-sensitive pickup device mounted to said supporting means for converting mechanical vibrations established therein by the interaction of said tool and said workpiece into an electrical signal, the magnitude of said electrical signal being proportional to the difference between the present orientation of the workpiece and the preferred orientation; and

means, responsive to the magnitude of said electrical signal,

for rotating said workpiece towards said preferred orienration.

7. The apparatus according to claim 6, wherein said rotating means comprises an electric motor; and the apparatus further comprises:

amplifying means for amplifying the magnitude of the electrical signal from said pickup device; and

control means, connected to the output of said amplifying means, including a bistable device having a predetermined threshold, for disconnecting the energizing potential applied to said motor whenever the magnitude of said amplified electrical signal exceeds said predetermined threshold thereby terminating further rotation of said workpiece.

8. The apparatus according to claim 7, further comprising:

means, connected to the input of said amplifying means, for

adjusting the sensitivity thereof to compensate for variations in the magnitude of said electrical signal caused.

9. The apparatus according to claim 6, wherein said rotating means comprises an electric motor; and the apparatus further comprises:

amplifying means for amplifying the magnitude of the electrical signal from said pickup device; and

control means, connected to the output of said amplifying means, including a bistable device having a predetermined threshold, for disconnecting the energizing potential applied to said motor whenever the magnitude of said amplified electrical signal falls below said predetennined threshold thereby terminating further rotation of said workpiece.

10. The apparatus according to claim 9, further comprising:

means, connected to the input of said amplifying means, for

adjusting the sensitivity thereof to compensate for variations in the magnitude of said electrical signal caused.

t: i IF 

1. A method of adjusting the orientation of a single-crystal workpiece, with respect to a cutting tool, so that said workpiece attains a preferred orientation, comprising the steps of: generating an electrical signal representative of the difference between the present orientation of the workpiece and said preferred orientation; and rotating said workpiece, with respect to said tool, under control of said electrical signal, until the workpiece attains said preferred orientation.
 2. The method according to claim 1, wherein the magnitude of said electrical signal decreases as the orientation of said workpiece approaches the preferred orientation; the method comprising the further steps of: amplifying said electrical signal in a control circuit including a bistable device having a predetermined threshold; and terminating the rotation of said workpiece, under control of said bistable device, whenever the magnitude of said amplified electrical signal falls below said predetermined threshold.
 3. The method according to claim 2, comprising the further step of: adjusting the amplifying sensitivity of said control circuit to compensate for variations in the magnitude of said electrical signal caused by variations in the relative movement of said workpiece and said tool; and by variations in the mechanical properties of said workpiece and said tool.
 4. The method according to claim 1, wherein the magnitude of said electrical signal increases as the orientation of said workpiece approaches the preferred orientation; the method comprising the further steps of: amplifying said electrical signal in a control circuit including a bistable device having a predetermined threshold; and terminating the rotation of said workpiece, under control of said bistable device, whenever the magnitude of said amplified electrical signal exceeds said predetermined threshold.
 5. The method according to claim 4, comprising the further step of: adjusting the amplifying sensitivity of said control circuit to compensate for variations in the magnitude of said electrical signal caused by variations in the relative movement of said workpiece and said tool; and by variations in the mechanical properties of said workpiece and said tool.
 6. Apparatus for automatically rotating a single crystal workpiece, with respect to a cutting tool, so that said workpiece attains a preferred orientation, which comprises: means for supporting said workpiece proximate said cutting tool; a vibration-sensitive pickup device mounted to said supporting means for converting mechanical vibrations established therein by the interaction of said tool and said workpiece into an electrical signal, the magnitude of said electrical signal being proportional to the difference between the present orientation of the workpiece and the preferred orientation; and means, responsive to the magnitude of said electrical signal, for rotating said workpiece towards said preferred orientation.
 7. The apparatus according to claim 6, wherein said rotating means comprises an electric motor; and the apparatus further comprises: amplifying means for amplifying the magnitude of the electrical signal from said pickup device; and control means, connected to the output of said amplifying means, including a bistable device having a predetermined threshold, for disconnecting the energizing potential applied to said motor whenever the magnitude of said Amplified electrical signal exceeds said predetermined threshold thereby terminating further rotation of said workpiece.
 8. The apparatus according to claim 7, further comprising: means, connected to the input of said amplifying means, for adjusting the sensitivity thereof to compensate for variations in the magnitude of said electrical signal caused.
 9. The apparatus according to claim 6, wherein said rotating means comprises an electric motor; and the apparatus further comprises: amplifying means for amplifying the magnitude of the electrical signal from said pickup device; and control means, connected to the output of said amplifying means, including a bistable device having a predetermined threshold, for disconnecting the energizing potential applied to said motor whenever the magnitude of said amplified electrical signal falls below said predetermined threshold thereby terminating further rotation of said workpiece.
 10. The apparatus according to claim 9, further comprising: means, connected to the input of said amplifying means, for adjusting the sensitivity thereof to compensate for variations in the magnitude of said electrical signal caused. 